Conventionally known as a large-size-screen television is a rear projection type television in which imaging light (projected light) is projected on a rear projection screen from a projector placed at the rear of the rear projection screen to display an image to viewers.
In such a rear projection type television, imaging light projected from the projector is spread and the spread light is projected on the rear projection screen, so that it is necessary to keep a certain distance between the projector and the rear projection screen. A problem with a television of this type, therefore, is that the space in the direction of depth is apt to become large.
In view of this problem, there has been proposed so far a rear projection type television in which, in order to make the space in the direction of depth smaller, a projector 5 is placed obliquely below a rear projection screen 10′ at the rear of it, as shown in FIG. 13, whereby the projector 5 is allowed to project imaging light LS obliquely and upwardly on the rear projection screen 10′.
Conveniently used as the rear projection screen 10′ in a rear projection type television as is shown in FIG. 13 is a rear projection screen comprising a total reflection Fresnel lens, capable of condensing imaging light projected obliquely from its rear (Japanese Laid-Open Patent Publication No. 208041/1986). The total reflection Fresnel lens herein denotes a lens having a plurality of prisms, where imaging light is refracted at the first plane (plane of refraction) of each prism and is then totally reflected at the second plane (plane of total reflection) of each prism to emerge toward the viewer's side.
In a rear projection screen comprising such a total reflection Fresnel lens, the light path of imaging light is adjusted by totally reflecting the imaging light. Therefore, the rear projection screen can attain high transmittance even when imaging light is obliquely incident on the rear projection screen at a large angle.
However, the conventional rear projection screen described above is confronted with the following problem. Namely, in an area of a Fresnel lens sheet 1′ constituting the conventional rear projection screen, on which area imaging light LS is incident at a small angle (an area close to a projector 5), a part of the imaging light LS refracted at the plane of refraction 3′ of each prism 2′ passes through the prism 2′ without being totally reflected at the plane of total reflection 4′ of the prism 2′ and becomes stray light LY, as shown in FIG. 14. The stray light LY thus produced is, as shown in FIG. 15, reflected at the plane of emergence 1b′ of the Fresnel lens sheet 1′ and returns to the incident side via a base 1a′; this light repeatedly causes incidence and emergence while passing through a plurality of the prisms 2′, and finally emerges from the plane of emergence 1b′. As shown in FIG. 15, the stray light LY emerging from the plane of emergence 1b′ in this manner comes out from a point that is different from the point from which the imaging light LS is totally reflected from the plane of total reflection 4′ of the prism 2′, after being refracted at the plane of refraction 3′ of the prism 2′, comes out as normal light LZ. The stray light LY, therefore, causes such troubles as the production of double images to decrease image visibility.